Dermot Mulroney will join his "My Best Friend's Wedding" co-star Julia Roberts in the film version of Tracy Letts' Pulitzer Prize-winning play August: Osage County, according to Deadline.com.

Dermot Mulroney

Mulroney will play Steve Heidebrecht, the fiancé of Karen Weston, played by Oscar nominee Juliette Lewis, who recently joined the cast along with Margo Martindale and Chris Cooper.

Martindale, who was Tony-nominated for her performance in the 2004 revival of Cat on a Hot Tin Roof, will be featured as Mattie Fae Aiken, sister of Violet, who will be played by three-time Oscar winner Meryl Streep ("The Iron Lady," "Doubt," "Julie & Julia"). Mattie Fae's husband, Charlie Aiken, will be played by Oscar winner Cooper (Of the Fields, Lately; "Adaptation").

As previously reported, Academy Award nominee Abigail Breslin (The Miracle Worker, "Little Miss Sunshine") will play 14-year-old Jean Fordham, the rebellious, pot-smoking daughter of Barbara, who will be played by Oscar winner Roberts ("Erin Brockovich"). Streep leads the family as Violet — mother to Barbara and Karen — a cancer-sick drug addict whose husband goes missing.

The darkly comic drama, which also won the Tony Award as Best Play, is a large ensemble piece (with three sisters at its core) set in a rambling house in Oklahoma. Letts also penned the screenplay. John Wells (TV's "E.R." and "Shameless") will direct. Additional casting will be announced.

The Weinstein Company film is a Jean Doumanian / Smokehouse production produced by George Clooney and Grant Heslov, Jean Doumanian and Steve Traxler. Claire Rudnick Polstein is the executive producer. Clooney is an Academy Award-winning actor ("Syriana") and an Academy Award-nominated filmmaker ("Good Night and Good Luck," "The Idea of March"). Heslov is an Academy Award-nominated producer/screenwriter ("Good Night and Good Luck," "The Ides of March"). Jean Doumanian Productions has has produced the Tony Award-winning productions of August: Osage County, The Book of Mormon and Death of a Salesman.

Clooney said in a previous statement, "Grant and I are thrilled to work on this project. It's already a great cast and great material and I can't think of anyone better than Harvey to put this all together. And we're particularly excited to work with our dear friend John Wells. It’s such a terrific group and we feel honored to help bring it to the screen."

Here's how TWC characterizes the picture: It "tells the dark, hilarious and deeply touching story of the strong-willed women of the Weston family, whose lives have diverged until a family crisis brings them back to the Midwest house they grew up in, and to the dysfunctional woman who raised them. Letts' play made its Broadway debut in December 2007 after premiering at Chicago's legendary Steppenwolf Theatre earlier that year. It continued with a successful international run."